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Astronomers Find a New Size of Black Hole

Astronomers announced yesterday that they had convincing evidence for a new class of black hole -- bigger than those formed from collapsed stars but much smaller than the gargantuan gravitational sinkholes found at the centers of galaxies.

Until now, black holes -- objects that have collapsed so tightly that not even light can escape their gravitational pull -- have come in two varieties: collapsed stars that have at most 10 times the mass of the sun, and the supermassive black holes that are believed to exist at the core of every galaxy, with masses up to that of a billion suns.

As in a Goldilocks-like tale, the new black hole is nicely though inexplicably between those ranges: its mass is at least that of 500 suns.

Now, after pinning down the existence of midsize black holes, astrophysicists have a new puzzle: explaining how the universe creates them.

''There is no way this object could form'' from the collapse of one star, because it is too big for that, said a member of the research team, Dr. Philip Kaaret, an astrophysicist at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

''I think this is going to be a very lively subject of future study,'' added Dr. Douglas Richstone, a professor of astronomy at the University of Michigan, who was not connected with the work but attended the Washington news conference where the findings were announced yesterday.

Astronomers estimated the size of the black hole from measurements of X-rays, which shoot out as matter falls into the abyss. A team of American and Japanese astronomers using NASA's Chandra X-Ray Observatory had spotted a bright X-ray beacon in the galaxy M82, about 12 million light-years from earth. They observed that the brightness of the beacon varied about 10 percent every 10 minutes. Such fluctuations are believed to result from vibrations in the disk of gas and dust swirling around black holes.

''It's the same as a bell,'' Dr. Kaaret said. ''These oscillations are particularly interesting, because black holes in our own galaxy make similar but faster oscillations.''

Just as lower-pitch rings emanate out of larger bells, the slower X-ray oscillations, combined with their brightness, indicate a larger black hole, somewhere between several hundred and 100,000 solar masses.

The laws of physics do not rule out midsize black holes, and in the 1980's Dr. Giuseppina Fabbiano of the Harvard-Smithsonian Center for Astrophysics observed moderately bright sources of X-rays in distant galaxies and suggested that they might come from such holes. Then, last year at a meeting of the American Astronomical Society's High Energy Astrophysics Division, two astrophysicists from Carnegie Mellon University reported observations that they said indicated a midsize black hole in M82. Separately, two astronomers at NASA's Goddard Space Flight Center said they had spotted three midsize black holes in other galaxies.

But all those claims were based on data from earlier satellites, which could not pinpoint where the X-rays were coming from or distinguish one midsize black hole from a group of star-size black holes. Chandra's sharper observations show five beacons of X-rays, not one, within M82. Four appear unextraordinary, possibly remnants of exploded stars or smaller black holes. The fifth, the brightest one, is the midsize black hole.

The black hole's location, 600 light-years from the center of M82, rules out the possibility that it is just a quiet version of the galactic variety. ''If it were a million-solar-mass black hole, I think it would sink to the center of the galaxy like a stone in honey,'' Dr. Richstone said. ''I think in this case it is clearly an intermediate-mass black hole.''

The M82 black hole, situated within a dense cluster of stars, may have grown by swallowing stars and star-size black holes. ''I think it is hundreds of stellar collisions that build up an object like this,'' Dr. Richstone said. Another possibility is that a large cloud of gas directly collapsed into a black hole.

Whatever the case, astronomers hope that by learning how midsize black holes form, they can gain clues about the origin of the supermassive black holes, also a mystery.